This invention relates to angular rate sensor devices in general and more particularly, to a ring laser gyro having means to compensate for temperature gradients and residual lock-in effects.
The ring laser gyro is a significant departure from prior art angular rate sensor devices. Conventional angular rate sensors employ a spinning mass to provide a reference direction. Sensors utilizing spinning masses have inherent problems among which are high drift rates, caused by friction, and unwanted torques. The ring laser gyro for the most part eliminates the undesirable characteristics of these prior art sensors. Its operation is based entirely upon optical and electronic phenomena wherein angular motion is measured by massless light waves circulating in a closed path.
U.S. Pat. Nos. 3,373,650 and 3,467,472 to Joseph E. Kilpatrick teaches a triangularly shaped ring laser resonant cavity defined by three corner mirrors. The triangular shape is preferred because it uses a minimum number of mirrors. A gas laser fills the laser cavity. The gas laser filling the cavity comprises helium and neon gas usually operating at one of two wavelengths, either 1.15 micrometers in the infrared spectral band or 0.63 micrometers in the visible wavelength region. Through a proper choice of the ratio of the two neon isotopes Ne.sup.20 and Ne.sup.22 in the gas mixture, two monochromatic beams are created. The two laser beams propagate in clockwise and counterclockwise directions around the triangular cavity following the same closed path. With no rotation about the input axis, the cavity lengths for the two beams are equal and the two optical frequencies are the same. Rotation in either direction causes an apparent increase in cavity length for the beam travelling in the direction of rotation and a decrease for the other beam. Since the closed optical path is a resonant cavity providing sustained oscillation, the wavelength of each beam must also increase or decrease accordingly. Rotation of the ring in either direction thus causes a frequency split and the two frequencies are unequal by an amount proportional to the rotation rate. At one mirror which is used as an output mirror, the clockwise and counterclockwise beams are extracted and heterodyned in a beam combiner to produce a heat frequency which is detected by two photodetectors. Both detectors sense the beat frequency caused by heterodyning of the two signal frequencies, which is a measure of rotation rate, but their outputs differ in phase by .+-.90.degree. depending on the direction of rotation. Hence the detector outputs contain information with respect to both the magnitude and direction of input rotation.
All ring laser gyroscopes are sensitive to temperature gradients across their line of symmetry. Such gradients affect the Langmuir flow. The Langmuir flow, caused by cataphoretic pumping between anode and cathode, is usually well-balanced by careful machining of the capillary bores that contain the glow discharge and by the utilization of two symmetrically placed glow discharges as well as by maintaining a constant current discharge in the two glow discharges by means of two active current regulators.
The ring laser gyroscopes of the prior art are extremely sensitive to temperature changes present in the environment or temperature changes caused by warmup. These temperature changes in prior art ring laser gyroscopes cause gradients across their plane of symmetry because the gyroscope block, as taught by the prior art, was unsymmetrical. As a result, output pulses appear although there has been no rotation about the input axis. In the prior art, the prevention of lock-in by mechanical dither necessitated an unsymmetrical block to exactly compensate undesired, dither-produced counts. Lock-in occurs at low input rotation rates, i.e., when the input rate falls below a certain critical or threshold value. In the lock-in region, a nonlinear relationship exists between the input and the output. Beyond the lock-in region, there is a substantial linear relationship between the input and output.
Prior art ring laser gyroscopes are mechanically dithered at a frequency of 100-500 Hz. Residual lock-in effects are evident in such gyroscopes. Such residual lock-in effects cause discrete nonlinearities in the input-output scale constant. Usually a pseudo-random dither motion is used to minimize the nonlinearities. Such randomness adds noise to the ring laser output if the compensating beam combiner does not exactly cancel the output from the ring laser. Such adjustment in prior art ring lasers is made by further adjusting the unsymmetry or offset--a very tedious adjustment because all optical adjustments have to be completely remade each successive time the offset is adjusted.
Prior art ring laser gyroscopes have beam combiners in which the convergence between the clockwise beam and the counterclockwise beam is fixed by tight manufacturing tolerances of the angles in the beam combiner. To maintain such tight tolerances is expensive and limits the freedom of choosing the mirror location when assembling the output mirror to the ring laser gyro.
Accordingly, it is an object of this invention to provide a ring laser gyro inherently insensitive to temperature changes.
It is also an object of this invention to provide a ring laser where the dither motion can be exactly cancelled without relocating the ring laser gyro with respect to the dither axis and where the convergence of the beams onto the output dual photo-detector can be adjusted to provide a 90.degree. phase difference between the outputs of the photo detectors.
It is a further object of this invention to provide a ring laser gyro inherently insensitive to residual noise effects caused by a pseudorandom dither motion.